Remove support for Python 3.5

[ilons]

As 3.5 approaches its end-of-life date (2020-09-13)[https://devguide.python.org/#status-of-python-branches], we should also drop support for it and stop building it. This would allow for us to pick up 3.9 being released on (2020-10-04)[https://devguide.python.org/#status-of-python-branches] and in the long run make for less workarounds for legacy.

[mjuenema]

That's fair enough, given that only a small number of downloads from PyPi are for Python 3.5.

Version	Downloads
3.4	4
2.7	44
3.5	409
3.8	534
3.6	4352
null	5204
3.7	6732

https://console.cloud.google.com/bigquery

SELECT 
   REGEXP_EXTRACT(details.python, r'^\d*\.\d*') python
  , COUNT(*) downloads
FROM `the-psf.pypi.downloads2020*`
WHERE file.project='terrascript'
GROUP BY python
ORDER BY downloads

[hugovk]

Please see PR #124 to drop 3.5, and #125 to test on 3.9-dev to make sure tests are passing for the full 3.9.0 release.

[ilons]

Great work @hugovk, much appreciated help with the move to Python 3.9.